Active control of acoustic radiation due to discontinuities on thin beams

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Abstract

Two experiments were conducted to study the active control of acoustic radiation
due to discontinuities on thin beams. One experiment investigated the radiation
from a clamped end condition and the other investigated the radiation from a
blocking mass. The beams were excited by subsonic flexural traveling waves which
"scattered" (or produced reflected and transmitted traveling and near-field waves)
when they encountered the discontinuity. This "scattering" produced supersonic
wave number components in the beam vibrational response which were responsible
for the acoustic radiation. The main purpose of these experiments was to control
the acoustic radiation from discontinuities on beams by actively changing the
characteristics of the "scattered" waves with control actuators.

In each experiment the system was disturbed by a harmonic, subsonic input from
a point force shaker. Control actuator( s) (in the form of shakers and piezoelectric
actuators) were attached to the beam near the discontinuity. Error microphone(s)
were positioned in the acoustic field which supplied an error signal to the digital
controller. The digital controller employed was the filtered-x version of the adaptive
LMS algorithm programmed on a dedicated signal processing board in a personal
computer.

An array of accelerometers was attached to the beam which were used to decompose
the complex amplitudes of an assumed displacement equation. By applying a spatial
Fourier transform to the displacement equation the wavenumber components present
in the beam displacement were calculated. This aided in the investigation of the
mechanism by which control of the acoustic field was affected.

Results from these experiments showed that large attenuations at the error microphones
were possible (as much as 50dB) along with global attenuation of the acoustic
field. The mechanism by which the control of the acoustic far-field was achieved was
demonstrated as a decrease in the supersonic wavenumber components in the beam
vibrational response.